East Asian Westerly Jet Stream Research Articles

Analysis of the Relationship between Upper-Level Aircraft Turbulence and the East Asian Westerly Jet Stream

The jet stream is a primary factor contributing to turbulence, especially for upper-level aircraft. This study utilized pilot reports and ERA5 data from 2023 to investigate the relationship between upper-level turbulence and the East Asian westerly jet (EAJ). The results indicate that approximately 45.9% of upper-level aircraft turbulence occurs within the jet stream, with the lowest proportion in August and the highest in January. Additionally, the strongest vertical wind shear (VMS) is found concentrated in the lower part of the jet stream core, particularly in the South–Down part of the jet stream, where upper-level aircraft turbulence occurs most frequently (27.1%). The most turbulent area is located between 30–40° N and 110–120° E, with the main air routes experiencing turbulence being the Henan sections of G212 and B208. From a seasonal perspective, there is less frequent occurrence of upper-level aircraft turbulence in summer and autumn but more in winter and spring. The EAJ volume increases with the strengthening of the jet core wind speed, with the jet core regions being most distinct at altitudes of 200~300 hPa. Meanwhile, the jet stream intensity index peaks at 70.6 m/s in January and reaches its lowest value of 7.1 m/s in August. The jet stream axis shifts southward in winter and northward in summer, reaching the southernmost position in December at 32.2° N and the northernmost position in August at 43.5° N. Furthermore, the VMS at turbulence points within the jet stream is higher than that at the turbulence points outside the jet stream, and the Richardson number (RI) is lower. Moreover, the temporal distribution of upper-level aircraft turbulence is primarily determined by the location and intensity of the jet stream, of which the jet stream intensity index provides guidance and thus serves as a reliable indicator.

Different impacts of the variations of western and eastern portions of the East Asian westerly jet stream on southern China rainfalls in Meiyu season

Variations of East Asian westerly jet (EAJ) drastically affect the rainfall in southern China (SC) during Meiyu season. Previous studies conventionally focused on the entire EAJ's variations and their impacts. In this study, we find that the variations of EAJ eastern section (EAJE) and western section (EAJW) are mostly independent of each other. Moreover, the independent components of EAJE and EAJW's variations have different impacts on the rainfall in SC. Southward-displaced and intensified EAJE (SI-EAJE) and southward-displaced and weakened EAJW (SW-EAJW), the first two leading modes of EAJ variations, are associated with increased rainfall anomalies centred over central and southern parts of SC, respectively.Associated with the SI-EAJE (SW-EAJW), anomalous cold cyclone is centred around the Japan Sea (central China). The associated anomalous westerly wind and northward-decreasing (northwestward-decreasing) temperature centred over the central (southern) SC together induce midtropospheric warm advection and thus positive rainfall anomalies centred over the central (southern) SC. The anomalous circulations also cause moisture convergences roughly collocated with the anomalous warm advections over the SC, which intensifies the rainfall anomalies.Warm sea surface temperature anomalies in the equatorial central-eastern Pacific during boreal spring induce meridional teleconnection along the coastal East Asia in Meiyu season through suppressing the convective activities in the northwestern Pacific. The resultant SI-EAJE and associated anomalous circulations increase the rainfall over the central SC, and vice versa. Dipole of increased (decreased) spring snow anomalies on the eastern (western) Tibetan Plateau suppresses (intensifies) convective activities in Meiyu season. The resultant SW-EAJW and associated anomalous circulations increase the rainfall in the southern SC.

Characteristics and circulation patterns for wet and dry compound day-night heat waves in mid-eastern China

Compound day-night heat waves (CohotEs) have more adverse effects on human health and ecosystem, and the consequences vary with humidity. This study classifies the CohotEs over Chinese mainland into wet and dry CohotEs, and compares their characteristics and associated circulation anomalies. Here, a populated region of mid-eastern China (MEC) with higher occurrence of both types of CohotEs is mainly focused. Results show that, during past six decades, wet CohotEs have higher frequency, longer duration and broader spatial scale, while dry CohotEs are less, shorter and more regional. Both types of CohotEs are associated with anticyclonic anomalies accompanied by the strengthening and northwestern expansion of western Pacific subtropical high (WPSH), leading to adiabatic heating with anomalous subsidence. The anomalies in wet CohotEs are influenced by the eastward expansion of East Asian westerly jet stream and northeastward expansion of the south Asia high. Anomalous southwesterlies transport additional water vapor from the tropics, leading to above-normal humidity that could reduce the amplitudes of anomalous subsidence but favor heat maintenance. By contrast, dry CohotEs are related to a more stable barotropic anticyclonic structure, and the anomalous signals are stronger in the middle-lower troposphere. Meanwhile, tropical cyclones appear over the South China Sea below 500 hPa, promoting the intensification of northwestward shifts of WPSH and anomalous northeasterlies over South China. So, lower humidity results from weakened northward water vapor transport could lead to faster heat dissipation and thus shorter duration for dry CohotEs. Our results emphasize the diversity of CohotEs, indicating that different types of CohotEs should be analyzed separately and multiple local and large-scale circulations need to be considered for better forecasts and predictions of CohotEs. • Characteristics of wet and dry compound hot extremes (CohotEs) are analyzed. • Circulation anomalies of wet and dry CohotEs are compared over mid-eastern China. • Anomalous anticyclone and western Pacific subtropical high (WPSH) cause CohotEs. • Wet CohotEs are influenced by eastward (westward) shifts of south Asia high (WPSH). • Dry CohotEs are related to shifts of WPSH and tropic cyclonic anomaly.

Effects of Suppressed Transient Eddies by the Tibetan Plateau on the East Asian Summer Monsoon

AbstractIt is known that the Tibetan Plateau (TP) can weaken the transient eddies (TEs) transported along the westerly jet stream. This study investigates the effects of the persistently suppressed TEs by the TP on the East Asian summer monsoon and the associated mechanisms using the NCAR Community Earth System Model. A nudging method is used to modify the suppression of the TEs without changing the steady dynamic and thermodynamic effects of the TP. The suppressed TEs by the TP weaken the East Asian westerly jet stream through the weakened poleward TE vorticity flux. On the one hand, the weakened jet stream leads to less (more) rainfall in northern (southern) East Asia by inducing anomalous moisture convergence, midtropospheric warm advection, and upper-level divergence, particularly in early summer when the eastward propagation of TE suppression by the TP is strong. On the other hand, the precipitation anomalies can shift the East Asian westerly jet stream southward and promote the moisture convergence in southern East Asia through latent heat release. Therefore, the persistent suppression of the TEs leads to a southward shift of the East Asian rain belt by a convective feedback, as it was previously found that the steady thermodynamic and dynamic forcings of the TP favored a northward shift of the rain belt. This study suggests that the anomalously weak TEs can lead to a rainfall change (more in the south, less in the north) over East Asia.

More frequent summer heat waves in southwestern China linked to the recent declining of Arctic sea ice

Southwestern China (SWC) has suffered from increasing frequency of heat wave (HW) in recent summers. While the local drought-HW connection is one obvious mechanism for this change, remote controls remain to be explored. Based on ERA-5 reanalysis, it is found that the SWC summer HWs are significantly correlated with sea-ice losses in the Barents Sea, Kara Sea and the Arctic pole. The reduction of Arctic sea ice can cause low pressure anomalies over the polar region due to increased heat-flux exchanges at the sea-air interface, which subsequently triggers southeastward Rossby wave trains propagating from northern Europe to East Asia that induce anomalous anticyclone over SWC. As a result, the North Pacific subtropical high extends westward, accompanied by divergent winds, decreased cloud cover and increased insolation in SWC, which leads to above-normal air temperatures there. In addition, the East Asian westerly jet stream is shifted northward, which enhances (reduces) the moisture convergence in North China (SWC), resulting in prominently drier soil in SWC. Therefore, the sea ice—forced changes in atmospheric circulation and surface conditions favor the occurrences of SWC summer HWs.

Possible influence of Arctic oscillation on precipitation along the East Asian rain belt during boreal spring

In this study, the possible influence of the springtime Arctic oscillation (AO) on precipitation along the East Asian rain belt has been studied for the period of 1979 to 2014. To capture the features of the large-scale variability of the atmospheric circulation and precipitation, singular value decomposition (SVD) analysis was performed. The domain for precipitation is from 21.25 to 33.75° N and 111.25 to 133.25° E, and the 1000 hPa heights are from 20° N northward. Prior to analysis, the El Nino-Southern Oscillation (ENSO) signals were linearly fitted and subtracted from the variables of interest. The first paired modes explain 51.1 % of the total squared covariance. The spatial feature of the pressure mode is almost identical to that of the positive AO pattern, while the precipitation mode displays an overall less-than-normal anomaly along the rain belt. The AO indices are highly correlated with the time coefficients of the pressure mode at a value of 0.91, and for the time coefficients of the precipitation mode, the correlation is −0.44, both of which are significant at the 99 % level. The regional atmospheric circulation anomalies in association with the negative phase AO mode display consistent changes, including the anomalous southerly winds and vapor flux convergence in the lower troposphere over East Asia, the stronger East Asian westerly jet stream, and the enhanced ascending air motion between 20 and 30° N. There are two possible mechanisms linking the AO and East Asian circulation and precipitation, i.e., the wave-trains along the westerly jet stream from North Africa to the Middle East and East Asia and the dipole of an anti-cyclone and a cyclone over the North Pacific.

Influence of internal decadal variability on the summer rainfall in Eastern China as simulated by CCSM4

The combined impact of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) on the summer rainfall in eastern China was investigated using CCSM4. The strongest signals occur with the combination of a positive PDO and a negative AMO (+PDO−AMO), as well as a negative PDO and a positive AMO (−PDO+AMO). For the +PDO−AMO set, significant positive rainfall anomalies occur over the lower reaches of the Yangtze River valley (YR), when the East Asian summer monsoon becomes weaker, while the East Asian westerly jet stream becomes stronger, and ascending motion over the YR becomes enhanced due to the jet-related secondary circulation. Contrary anomalies occur over East Asia for the −PDO+AMO set. The influence of these two combinations of PDO and AMO on the summer rainfall in eastern China can also be observed in the two interdecadal rainfall changes in eastern China in the late 1970s and late 1990s.

Contribution of the phase transition of Pacific Decadal Oscillation to the late 1990s' shift in East China summer rainfall

AbstractBased on our previous study, the interdecadal changes in summer rainfall over East China in the late 1990s are further explored here. The increased local rising motion is implicated as the dominant factor of increased rainfall in the lower Huang‐Huai River valley (LHR). Both the observation and numerical experiments using Community Atmosphere Model, version 4 suggest that the negative Pacific Decadal Oscillation (PDO) mode can result in rising anomalies and thus more rainfall in the LHR. The East Asian westerly jet stream (EAWJS) is suggested as a bridge to link the Pacific sea surface temperature anomalies and East Asian summer rainfall. Model results reveal that the negative PDO mode can lead to significant easterly anomalies over East Asia. As a result, the EAWJS is weakened and shifts poleward, which coincides with observed changes in EAWJS after the late 1990s. In addition, weakened and poleward shifted EAWJS can result in an anomalous ascending motion to its south (in the LHR) by modulating the jet‐related secondary meridional‐vertical circulation. Consequently, rainfall increased in the LHR after the late 1990s. Besides, the positive Atlantic Meridional Oscillation can only induce insignificant changes over East Asia and partly counteract the negative PDO effect there.

A connection between the tropical Pacific Ocean and the winter climate in the Asian‐Pacific region

AbstractThe impact of the tropical Pacific sea surface temperature (SST) anomaly on the winter mean surface air temperature (SAT) in the Asian‐Pacific region is investigated during the period from 1948 to 2008 using both observations and a linear baroclinic model (LBM). A singular value decomposition (SVD) analysis is conducted between the 500 hPa geopotential height (Z500) over the Northern Hemisphere and the SST over the tropical Pacific Ocean to obtain the large‐scale atmospheric patterns related to tropical Pacific SST. Focus is given to the second pair of SVD mode (SVD2) which bears some similarities in the Z500 field to the Arctic Oscillation over the North Atlantic sector and can impact the SAT over a larger area of Asian‐Pacific. In the winter of a positive SVD2 the SAT over the midlatitude to high‐latitude Asian continent, the Arctic Ocean, the Indian Ocean, and the western subtropical Pacific Ocean tends to be warmer than normal, while the North Pacific Ocean around the Bering Strait is abnormally cold, and vice versa. Examination of the associated surface general circulation shows that a positive SVD2 tends to shift the Siberian High southward and the Aleutian Low eastward resulting in anomalous weak pressure gradient between the Asian continent the North Pacific. Anomalous positive sea level pressure anomalies around Japan and southerly wind along the east coast of the Asian continent are observed. At the same time, the East Asian trough at midtroposphere becomes weaker than normal and the East Asian westerly jet stream is increased in magnitudes and shifted northward. The analysis of the wave activity flux and result of idealized numerical experiments show a possible influence of the western tropical Pacific SST forcing on the SVD2.

Impacts of Autumn Arctic Sea Ice Concentration Changes on the East Asian Winter Monsoon Variability

Abstract The present study investigated the impacts of autumn Arctic sea ice concentration (SIC) changes on the East Asian winter monsoon (EAWM) and associated climate and circulation on the interannual time scale. It is found that the Arctic SIC anomalies have little impact on the southern mode of EAWM, but the northern mode is significantly associated with both western and eastern Arctic SIC anomalies. When there is less (more) SIC in eastern (western) Arctic, the EAWM tends to be stronger. The concurrent surface air temperature anomalies are induced both locally due to the direct effect of ice cover and in remote regions through anomalous wind advection. Analysis showed that eastern Arctic SIC anomalies have a larger effect on local atmospheric stability of the lower troposphere than western Arctic SIC anomalies. Winter temperature over the midlatitudes of East Asia is lower when there is more (less) SIC in the western (eastern) Arctic. The atmospheric response to the Arctic SIC anomalies is dominantly barotropic in autumn, and changes to baroclinic over the midlatitudes of Asia, but remains barotropic in other regions in winter. The mid- to high-latitude circulation systems, including the Siberian high, the East Asian trough, and the East Asian westerly jet stream, play important roles in connecting autumn Arctic SIC anomalies and the northern mode of the EAWM variability. No obvious concurrent sea surface temperature anomalies accompany Arctic SIC variations on the interannual time scale, indicating that the Arctic SIC anomalies have independent impacts on the East Asian winter climate.

Changes in Moisture Flux over the Tibetan Plateau during 1979–2011 and Possible Mechanisms

Abstract Changes in moisture as represented by P − E (precipitation − evapotranspiration) and the possible causes over the Tibetan Plateau (TP) during 1979–2011 are examined based on the Global Land Data Assimilation Systems (GLDAS) ensemble mean runoff and reanalyses. It is found that the TP is getting wetter as a whole but with large spatial variations. The climatologically humid southeastern TP is getting drier while the vast arid and semiarid northwestern TP is getting wetter. The Clausius–Clapeyron relation cannot be used to explain the changes in P − E over the TP. Through decomposing the changes in P − E into three major components—dynamic, thermodynamic, and transient eddy components—it is noted that the dynamic component plays a key role in the changes of P − E over the TP. The thermodynamic component contributes positively over the southern and central TP whereas the transient eddy component tends to reinforce (offset) the dynamic component over the southern and parts of the northern TP (central TP). Seasonally, the dynamic component contributes substantially to changes in P − E during the wet season, with small contributions from the thermodynamic and transient eddy components. Further analyses reveal the poleward shift of the East Asian westerly jet stream by 0.7° and poleward moisture transport as well as the intensification of the summer monsoon circulation due to global warming, which are shown to be responsible for the general wetting trend over the TP. It is further demonstrated that changes in local circulations that occur due to the differential heating of the TP and its surroundings are responsible for the spatially varying changes in moisture over the TP.

Distinguishing Interannual Variations of the Northern and Southern Modes of the East Asian Winter Monsoon

Abstract The East Asian winter monsoon (EAWM)-related climate anomalies have shown large year-to-year variations in both the intensity and the meridional extent. The present study distinguishes the interannual variations of the low-latitude and mid- to high-latitude components of the EAWM to gain a better understanding of the characteristics and factors for the EAWM variability. Through composite analysis based on two indices representing the northern and southern components (modes) of the EAWM variability, the present study clearly reveals features unique to the northern and southern mode. The northern mode is associated with changes in the mid- to high-latitude circulation systems, including the Siberian high, the Aleutian low, the East Asian trough, and the East Asian westerly jet stream, whereas the southern mode is closely related to circulation changes over the global tropics, the North Atlantic, and North America. A strong northern mode is accompanied by positive, negative, and positive surface temperature anomalies in the Indochina Peninsula, midlatitude Asia, and northeast Russia, respectively. A strong southern mode features lower temperature over tropics and higher temperature over mid- to high-latitude Asia. While the southern mode is closely related to El Niño–Southern Oscillation (ENSO), the northern mode does not show an obvious relation to the tropical sea surface temperature (SST) change or to the North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) on the interannual time scale. Distinct snow cover and sea ice changes appear as responses to wind and surface temperature changes associated with the two modes and their effects on the EAWM variability need to be investigated in the future.

Impact of the quasi‐biweekly oscillation over the western North Pacific on East Asian subtropical monsoon during early summer

The impact of quasi‐biweekly oscillation (QBWO) over the western North Pacific on East Asian summer monsoon (EASM) is investigated. The life cycle of QBWO is divided into eight phases defined by the two leading principal components (PC1 and PC2) of an empirical orthogonal function analysis. Subtropical rainfall shows significant changes, with a northwestward propagation of convection from equatorial regions to the South China Sea (SCS). The most significant variations occur in QBWO phases 3 and 4 (enhanced convection over SCS) and phases 7 and 8 (reduced convection over SCS). The East Asia Mei‐yu significantly decreases in QBWO phases 3 and 4 but increases in phases 7 and 8. The QBWO influences EASM through modulating the subtropical monsoon flow and extratropical circulation. The response of lower tropospheric atmosphere to QBWO shows a northwestward propagation and a downstream wave train that extends northward into the western North Pacific, modulating the SCS monsoon trough and the EASM flow associated with moisture transportation. The mid‐tropospheric extratropical circulation and the western Pacific subtropical high also show obvious changes accompanying QBWO evolution, resulting in circulation patterns associated with cold air activity. Moreover, changes with QBWO are found in the upper tropospheric East Asian westerly jet stream and the South Asian high, and these changes contribute to upper level divergence over subtropical East Asia.

Contributions of natural and anthropogenic forcings to the summer cooling over eastern China: An AGCM study

We estimate the contributions of natural and anthropogenic forcings to the observed summer cooling over the Eastern China through a set of controlled experiments of an atmospheric general circulation model. The control run is forced by historical natural and anthropogenic agents, as well as by the Hadley Center global sea surface temperature. The model suggests a weak influence by the solar radiation change on this cooling, but a significant contribution by the sulfate aerosol to this cooling, through both dynamic and thermal processes. The inclusion of the sulfate aerosol induces a positive gradient of air temperature in the middle‐upper troposphere, which results in a northward shift in the 200 hPa East Asian westerly jet stream and an increase of the East Asian summer monsoon, leading to more cloud cover and precipitation in the Eastern China therefore surface cooling over the region.

Relationship between stationary planetary wave activity and the East Asian winter monsoon

The variability of both the stationary planetary wave activity and the East Asian winter monsoon is strongly associated with the thermal contrast between oceans and landmasses. In this study, we explore the interannual relationship between the monsoon and the wave activity defined by an index of the difference in the divergence of Eliassen‐Palm flux between 50°N at 500 hPa and 40°N at 300 hPa. It is found that, compared to the winters of low wave activity, the equatorward propagation of planetary waves in the middle and upper troposphere is stronger in the high wave activity winters. During these high activity winters, the upward wave propagation from the troposphere into the stratosphere becomes weaker. This is accompanied by a smaller perturbation in the polar vortex, which tends to be colder and stronger. In the meantime, the East Asian westerly jet stream, the East Asian trough, the Siberian high, and the Aleutian low all become weaker apparently. In particular, the weakening of the Siberian high and the Aleutian low decreases the northeasterly wind over East Asia, leading to a warming condition in the region especially in northeastern Asia. A further analysis reveals that the zonal wavenumber‐2 pattern of planetary waves contributes dominantly to the variability of the East Asian winter monsoon.

Associations among the Components of the East Asian Summer Monsoon System in the Meridional Direction

The East Asian summer monsoon is characterized by strong interactions among its components in the meridional direction. The atmospheric convection over the Philippine Sea (PSAC), the western North Pacific subtropical high (WNPSH) and the East Asian westerly jet stream (EAJ) are all closely related to the summer rainfall in East Asia. In this study, we examined the relationship on the interannual timescale among these factors, by using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Re-analysis data and satellite-observed outgoing long-wave radiation (OLR) data for the 20-year period from 1979 to 1998. It is found that the PSAC-EAJ relation is weak in June, but strong in July and August. Similar difference exists in the relationship between the PSAC and convective activity along the East Asian summer rainy belt. Corresponding to enhanced PSAC, the EAJ is weakened in July and strengthened in August, and tends to exhibit a slight poleward displacement in both months. All these variations in the EAJ intensity and meridional displacement, on the other hand, correspond to suppressed convection along the East Asian summer rainy belt. Finally, the monthly difference in the PSAC-EAJ relation is interpreted by the role of vertical shear of zonal wind. The easterly shear in July and August over the Philippine Sea excites external modes, which are necessary for the tropical-extratropical teleconnection mechanism according to previous numerical studies, but the neutral shear in June is inefficient in exciting external modes.